The present invention relates to a memory device using an electrically rewritable nonvolatile semiconductor memory as a storage medium.
A magnetic disk device is the mainstream of a file data storage device in an information processing system such as a personal computer, or the like, at present. The magnetic disk device has a lot of merits such as low price, large capacity, high reliability, high performance, etc. and is most popular. With the gradual increase of demands for reduction in size, improvement in portability, reduction in consumed electric power, etc., a semiconductor memory device using a semiconductor as a recording medium has begun to be highlighted because the semiconductor memory device is more excellent in portability and low consumed electric power than the magnetic disk device.
A file device using a nonvolatile memory as a recording medium is disclosed in JP-A-5-27924. This technique concerns a memory device using a flash memory as a recording medium, and particularly discloses a system in which the life of a flash memory limited in the number of times of erasing required for rewriting is prolonged. Specifically, the number of times by which erasing is allowed is averaged for all regions in a file storage device which is apt to be rewritten locally, so that the apparent life is prolonged.
In the aforementioned invention, the number of times of erasing is used as an indicator for expressing deterioration of each flash memory cell and considered on the basis of the guaranteed value of the erasable number of times specified as characteristic data of the flash memory. The guaranteed value of the erasable number of times specified for each of various flash memories is, however, an average value or a minimum value in terms of characteristic, while the number of times by which erasing is allowed in practice varies according to the individual memory cells. That is, all the cells do not necessarily become unusable even when the number of times by which erasing is allowed reaches the guaranteed value. Accordingly, while the number of times by which erasing is allowed may be counted as an indicator for deterioration of each cell, there is some case where it cannot be said that memory cells are used most efficiently.
Therefore, an object of the present invention is to provide a method or semiconductor memory device in which deterioration of a memory can be grasped without consciousness of the guaranteed number of times and the current number of times by which the aforementioned electrically erasable/rewritable nonvolatile memory is allowed to be erased/rewritten, and, further, to provide a memory management method or semiconductor memory device using the same.
Another object of the present invention is to provide a method or semiconductor memory device in which the aforementioned electrically erasable/rewritable nonvolatile memory can be used up to the detected actual erasable/rewritable limit.
A further object of the present invention is to provide a method or semiconductor memory device for detecting the number of times by which the aforementioned electrically erasable/rewritable nonvolatile memory is allowed to be actually erased/rewritten.
In order to achieve the above objects, according to the present invention, in a semiconductor memory device using an electrically rewritable nonvolatile memory as a storage medium, there are provided: an erasing time measurement means for measuring erasing time required for erasing of the nonvolatile memory; a characteristic detection means for comparing the erasing time measured by the erasing time measurement means with an erasing time reference value to thereby determine environmental characteristic; a writing time measurement means for measuring writing time required for writing of the nonvolatile memory; a correction means for correcting the writing time measured by the writing time measurement means on the basis of the environmental characteristic determined by the characteristic detection means; and a nonvolatile memory deterioration determining means for detecting a degree of deterioration in a rewriting region of the nonvolatile memory on the basis of the corrected writing time.
Further, according to the present invention, in a semiconductor memory device using an electrically rewritable nonvolatile memory as a storage medium, there are provided: an electric source voltage detection means for detecting an electric source voltage value applied to the nonvolatile memory; a temperature detection means for detecting an ambient temperature of the semiconductor memory device; a characteristic detection means for determining environmental characteristic from detection values of the electric source voltage detection means and the temperature detection means; a writing time measurement means for measuring writing time required for writing of the nonvolatile memory; a correction means for correcting the writing time measured by the writing time measurement means on the basis of the environmental characteristic determined by the characteristic detection means; and a nonvolatile memory deterioration determining means for detecting a degree of deterioration in a rewriting region of the nonvolatile memory on the basis of the corrected writing time.
Further, according to the present invention, in a semiconductor memory device using an electrically rewritable nonvolatile memory as a storage medium, there are provided: an erasing time measurement means for measuring erasing time required for erasing of the nonvolatile memory; a writing time measurement means for measuring writing time required for writing of the nonvolatile memory; a characteristic detection means for comparing the writing time measured by the writing time measurement means with a writing time reference value to thereby determine environmental characteristic; a correction means for correcting the erasing time measured by the erasing time measurement means on the basis of the environmental characteristic determined by the characteristic detection means; and a nonvolatile memory deterioration determining means for detecting a degree of deterioration in a rewriting region of the nonvolatile memory on the basis of the corrected erasing time.
Further, according to the present invention, in a semiconductor memory device using an electrically rewritable nonvolatile memory as a storage medium, there are provided: an electric source voltage detection means for detecting an electric source voltage value applied to the nonvolatile memory; a temperature detection means for detecting an ambient temperature of the semiconductor memory device; a characteristic detection means for determining environmental characteristic from detection values of the electric source voltage detection means and the temperature detection means; an erasing time measurement means for measuring erasing time required for erasing of the nonvolatile memory; a correction means for correcting the erasing time measured by the erasing time measurement means on the basis of the environmental characteristic determined by the characteristic detection means; and a nonvolatile memory deterioration determining means for detecting a degree of deterioration in a rewriting region of the nonvolatile memory on the basis of the corrected erasing time.
Further, according to the present invention, in the above-mentioned semiconductor memory device, there are further provided: a deterioration-degree storage means for storing a degree of deterioration in every region detected by the nonvolatile memory deterioration determining means while the degree of deterioration in each of the regions is divided into a plurality of stages; a deterioration degree comparing means for comparing a degree of deterioration determined at the time of rewriting of each region with the degree of deterioration determined at the time of previous rewriting and stored in the deterioration-degree storage means; a low-deterioration-degree-region retrieval means for detecting a region having the lowest degree of deterioration among all regions; stored data of a region in which progression of deterioration is found as a result of comparison executed by the deterioration degree comparing means; and a data exchange means for exchanging the stored data in a region detected by the a low-deterioration-degree retrieval means.
Further, according to the present invention, in the above-mentioned semiconductor memory device, when the corrected erasing time or the corrected writing time exceeds a predetermined value, the nonvolatile memory deterioration determining means makes a determination that the degree of deterioration is in an unusable state and treats the maximum one of the degree of deterioration divided into the plurality of stages as the degree of deterioration in the unusable state to thereby prohibit data storage thereafter.
Further, according to the present invention, in the above-mentioned semiconductor memory device, the deterioration degree detection means is provided in the inside of the electrically rewritable nonvolatile memory.
Further, according to the present invention, in an information processing system comprising: a central processing unit for performing processing of program or data; a main memory for temporarily storing the program or data to be used by the central processing unit; a basic program memory which stores programs at the time of start and programs such as basic input/output instructions; a display control means and a display means for displaying a state of operation, a result of process, and so on; an input means for inputting a processing request; and a communication control means for performing data exchange or the like with other information processing systems; the aforementioned semiconductor memory device is provided as an auxiliary memory device.
The nonvolatile memory as a recording medium in a storage device is rewritten by region or chip having various data. An erasing operation and a writing operation are required for rewriting the nonvolatile memory. In each of the operations, inputting or controlling signals determined in accordance with the memory and a certain degree of time are required. Further, most of nonvolatile memories have properties in which the change of the time required for erasing operation is relatively small and the change of the time required for writing operation is very large with respect to the progress of deterioration or, conversely, the change of the time required for writing operation is relatively small and the change of the time required for erasing operation is very large with respect to the progress of deterioration.
Using these properties, the aforementioned writing time measurement means or the erasing time measurement means measures the change of the writing time or easing time of the nonvolatile memory as a recording medium so that a substantial degree of deterioration can be detected. That is, the degree of deterioration of every cell can be obtained while the scattering of characteristic is taken into account. Accordingly, not only reliability can be improved but also the memory can be used until the life of the memory approaches its limit.
Further, the nonvolatile memory is apt to be influenced by environmental conditions such as an electric source voltage value, an ambient temperature, etc. The time required for easing operation and the time required for writing operation correlate, to some degree, with the way of reception of the influence of such environmental conditions. When this property is used, for example, in the case where the change of the time required for erasing operation is relatively small and the change of the time required for writing operation is very large, the change of the time required for erasing operation can be regarded as the influence of such environmental conditions. When the time required for writing operation is corrected on the basis of the change of the time required for erasing operation, the influence of such environmental conditions can be eliminated so that deterioration can be judged.
Further, the nonvolatile memory deterioration determining means may measure the environmental conditions such as an electric source voltage value, an ambient temperature, etc. directly to thereby correct the change of the writing time or erasing time to judge deterioration. As a result, accuracy in the degree of deterioration is improved, so that this measure is of practical use. Further, this measure can be applied to a memory having properties in which both the time required for writing operation and the time required for erasing operation change remarkably because of deterioration.
When averaging the degree of deterioration in respective cells is attained on the basis of the degree of deterioration detected by the aforementioned means, the reliability and prolongation of life are improved greatly.
Further, when the aforementioned semiconductor memory device is used as an auxiliary storage device in an information processing system, the reliability of the information processing system per se can be improved and the life thereof can be prolonged.